This invention relates to the clarification of liquids containing suspended solids.
Sewage and other waste water treatment facilities must be designed to clarify liquids which contain suspended solids. In order to do this efficiently, the designer of such facilities must balance a flow stream so as to obtain efficient settling, yet maximize throughput.
Improved efficiency in removing the settleable solids from a waste liquid stream can be used either to increase throughput or to decrease facility size for a given throughput. In either case, for even small efficiency increases, the dollar savings can be great over the lifetime of the facility.
The most common method for purifying domestic sewage and many industrial wastewaters is by employing the activated sludge process. This process consists of maintaining an environment that fosters a flocculant biological mass (activated sludge) that is capable of removing impurities from wastewater flow. The impurities are taken up by the activated sludge as a food source and accumulated as additional biological mass.
In order to accomplish the purification process, the biological mass must be capable of removing the impurities from the waste flow and the process must be designed and operated so as to maintain the activated sludge in a physiological state whereby it can effectively be removed from the wastewater flow by gravitational settling.
The activated sludge process is accomplished in two phases. The first is an activated sludge phase wherein the biological mass is mixed and aerated with the incoming wastewater (to form a mixed liquor) in a manner that will accomplish the uptake of impurities and maintain the proper physiological state of the biological mass. The second phase is a separation phase, whereby a quiescent low-turbulence condition is maintained to permit the gravitational settling of the biological mass. The clarified liquid is generally removed at the surface and then either subjected to further treatment or disinfected and discharged to surface waters without additional treatment.
The concentrated activated sludge is returned to the incoming flow. Since the uptake of the impurities as food results in biological growth, a portion of the concentrated sludge must be removed or "wasted" from the process. The efficiency of the treatment process depends on a high level of uptake of the impurities in the incoming flow by the biological mass and efficient separation of the biological mass from the treated liquid.
Solids separation is not only an expensive but also a difficult part of wastewater treatment. The separation phase is also often the principal source of both process and mechanical problems.
The conventional practice in activated sludge process design and operation has been to accomplish the activated sludge phase in a vessel wherein mixing and aeration is provided by diffused aeration. The mixed liquor is then piped to a second vessel where the separation phase is accomplished by gravitational consolidation of the solids under quiescent conditions. The settled solids are mechanically scraped to an outlet where they are returned to a point of mixing with the incoming sewage by pumping. The solids retention time in the separation phase generally amounts to several hours. During this period, the solids are neither fed or aerated. This results in a degeneration of the biological quality of the activated sludge and hence a reduction in efficiency of the treatment process.
Another process design employs velocity of flow for mixing and aeration. This modification is generally designed as an orbital configuration. There are several means employed for providing aeration and mixing in the orbital aeration tank. Each method involves the use of mechanical energy input to mix, aerate and maintain the orbital flow. The circuitous channel can be designed in various configurations including elongated or circular. Some modifications employ a pumping system or impeller to create the circuitous flow. Other designs employ a surface motor driven aerator having a vertical axis. The most common design employs an aerator having a horizontal axis normal to the direction of flow, with the aerator extending across the channel. In all cases the energy input is to impart momentum to the flow. All aeration systems in activated sludge processes employing an orbital flow pattern must impart sufficient velocity to the orbital flow to ensure good mixing of the biological solids with the incoming flow, to prevent deposition of the solids and to provide adequate oxygen transfer to the flow. Manufacturers and designers employ a plurality of aerator designs and the placement of the aerators in the aeration tank. Like the standard activated sludge process, the separation phase is generally accomplished in a separate clarification vessel.
More recently, designs wherein the mixing and aeration steps of the activated sludge process have been combined with the clarification step into a single unit have been proposed in order to minimize facility costs and improve efficiency.
One of these, the "intrachannel" clarifier, has been designed for example as disclosed in U.S. Pat. No. 4,303,516 to Stensel et al. In this patent it is taught to use an integral clarifier in an endless flowing circuitous channel, wherein a solids containing liquid stream is split, the first portion being allowed to enter the upstream end of the clarifier. The clarifier is, in essence, a basin within the outer channel. Outlets from the clarifier are formed along the bottom thereof so that solids which collect there can be scraped to the outlets and back into the second portion of the split stream which flows under the clarifier. Clarified liquid is removed from the surface of the interchannel clarifier.
While U.S. Pat. No. 4,303,516 provides an improvement by forming a clarifier integrally in an orbital flow stream, this design nevertheless provides no turbulence protection in the quiescent zone to ensure that the liquid being removed will not vary in quality when turbulence due to flow changes occurs at the clarifier inlet. Further, this design is not appropriate for use in a non-flowing environment, for example in a system where mixing and aeration occur by diffusing air into the liquid rather than achieving this by imparting flow to the liquid.
Improvements to the intrachannel clarifier are also described in Cerwick, U.S. Pat. Nos. 4,446,018 and Morrow, 4,455,239.
In U.S. Pat. Nos. 4,446,018, the solids scraping mechanism required in the clarifier of 4,303,516 is eliminated by replacing the flat bottom thereof with angled baffles, so that solids which settle in the integral clarifier will simply settle back into the main flow stream flowing underneath the clarifier. Inlet flow is once again directed into the "quiescent" zone from which clarified liquid is taken with minimal turbulence interruption within the clarifier itself. The design of U.S. Pat. No. 4,446,018 must also be used in flowing systems.
In U.S. Pat. No. 4,455,239 a further improvement is introduced whereby an intrachannel clarifier design is modified such that (1) it can be used in nonflowing systems since it has an integral impeller to impart sufficient momentum to the liquid to cause it to pass through the clarifier, and (2) the inlet stream to the clarifier is separated from the quiescent zone by a series of baffles, which, as in U.S. Pat. No. 4,446,018 form the bottom of the quiescent zone.
Even in light of these improvements, however, drawbacks remain in the design of these intrachannel clarifiers. In order to maximize clarifying efficiency, turbulence and horizontal liquid flow in the quiescent zone must be minimized. In early designs such as U.S. Pat. Nos. 4,303,516 and 4,446,018 no protection from flow variations and resulting turbulence is provided between the quiescent zone and the means to remove clarified liquid. In U.S. Pat. No. 4,455,239 the quiescent zone is isolated by a single layer of baffles which in and of themselves create turbulence as liquid passes from the main flow channel into the quiescent zone. Thus, while horizontal flow is decreased by the baffles, turbulence is created and settling efficiency lost.
Furthermore, the upper face of the baffles is so near to being horizontal that solids accumulate on the baffles where they decompose and rise to the surface as floating masses, interfering with clarification and creating an unsightly condition.
In all solids separation processes, there must be a consolidation of solids in the lower part of the clarifier. In designs where the clarification is accomplished in a separate vessel, considerable depth must be provided for the consolidation of the solids. Previous intra-channel clarifiers have been relatively shallow and without provisions in the design to overcome the deficiency in solids consolidation depth. This reduces clarification efficiency.
In the U.S. Pat. No. 4,455,239 design, flow through the integral clarifier must be maintained at a sufficient rate to ensure that no solids settle in the clarifier itself. If this flow could be controlled, settling efficiency could be even further enhanced. In all of these designs, no provision is made for responding to changes in solids concentration and settleability at the clarifier inlet. It would be advantageous if variations in feed solids concentration and quality to the clarifier could be better managed in order to optimize solids separation at various process conditions.
Thus, it is an object of the invention to improve the solids separation settling efficiency of an intrachannel clarifier by designing a unit through which controlled flow rates can be achieved, which design provides a baffle arrangement and operator controlled turbulence, whereby solids can be rapidly returned to the underflow, minimizing the depth requirements for solids consolidation. The design also provides for minimizing both horizontal and vertical turbulence within the quiescent zone, and for the concentrating of solids to be wasted from the process.